1. Field of the Invention
This invention relates to the field of electronics. More exactly, this invention relates to solid state VMOS-FET electronic devices. More particularly, but without limitation thereto, this invention describes a VMOS-FET IMPATT diode pulse bias circuit designed to provide Gallium Arsenide (GaAs) double drift Impact Avalanche Transit Time (IMPATT) diodes with a specified operating voltage and current under required pulse conditions.
2. Description of the Prior Art
In the past, IMPATT diode pulse bias circuits have been constructed using discrete power bipolar transistors as the output device, and the pulse forming network has been built using discrete bipolar switching transistors. The primary disadvantage of this type of design is the limitation due to the selected output transistor. The bipolar transistor exhibits a maximum power efficiency when operated into saturation, but the saturation operation is not achievable in this application due to the necessity of achieving fast rise and fall times for the voltage/current pulses. The bipolar transistor will not switch quickly under saturated operation because of the device energy storage characteristics. Once the bipolar transistor is into saturation, the base circuit of the transistor capacitively stores energy. This stored energy must be dissipated during turn off, thus increasing the turn off time of the saturated bipolar transistor. To remedy this problem, the bipolar transistor is operated in the switching mode to minimize energy storage problems. In this way, fast rise and fall time are achievable, but at the cost of energy efficiency. During the switching mode of operation, a larger voltage is seen across the output transistor than during the saturation mode. This increase indicates that the device is no longer operating in its most efficient mode. Similar energy storage and efficiency problems can also be found in the pulse forming network when discrete bipolar devices are used. Saturated operation must be closely monitored to alleviate pulse distortion in the pulse forming network.
N-channel power VMOS-FETs have also been used as output transistors to control the current flow to IMPATT devices. The n-channel VMOS-FET transistor requires a special driver circuit when used in the source follower configuration, which is required for this application. The problem with this configuration is the associated transistor gate drive circuitry. The gate drive circuit must either be transformer coupled to the transistor, or must have a switching network connected between the transistor gate and source. The transformer coupling scheme is usable for applications where voltage rise and fall times can exceed 50 nanoseconds. This slow rise/fall time generally makes this circuit configuration usable ony for pulse widths in excess of one microsecond. The switching network scheme allows voltage rise/fall times faster than 50 nanoseconds, but the pulse width generally cannot exceed 500 nanoseconds due to problems with the switching network remaining stable. When the switching network becomes unstable, the VMOS-FET transistor begins to shut off if the gate-to-source voltage decreases. This situation does not allow the proper current to flow to the IMPATT diode and, in turn, the IMPATT diode does not produce the required output radio frequency (RF) power.